The digital still camera, often referred to by the abbreviation DSC, is at present one of the most common devices employed for acquiring digital images. The fact that both sensors of ever greater resolution and low-priced digital signal processors (DSPs) of low power consumption are readily available in commerce has led to the development of digital still cameras capable of acquiring images of very high resolution and quality.
Just like their traditional counterparts, however, these cameras still have a rather limited response when it comes to acquiring real scenes containing zones differing from each other by considerable illumination contrasts.
The sensitivity of an image-acquisition instrument to the light-intensity variations contained in a given scene is measured in terms of exposure latitude or dynamic range. This parameter represents the greatest ratio between the maximum and the minimum light intensity contained in the given scene to which the instrument is capable of responding.
The dynamic range of a digital still camera is determined by the characteristics of the sensor and by the format of the data after the analog/digital conversion.
The sensor, no matter whether it is of the CCD or the CMOS type, is an integrated device comprising a matrix of photosensitive cells, each of which provides an output voltage proportional to the quantity of light that strikes it during the acquisition time.
The curve representing the response of a photosensitive cell to the quantity of incident light consists of a threshold region, a saturation region and a linear region comprised between the threshold region and the saturation region (FIG. 2a).
The threshold region, which corresponds to the tract 6 of the curve shown in FIG. 2a, is characterized by the fact that in this region the photoelectric cell produces either very small or zero variations of its output voltage in response to variations of the quantity of incident light. It is therefore essential that during the acquisition time there should be present a minimum (threshold) quantity of incident light (or photons) to generate an output voltage that prevails over the noise voltage, a part of which will be present in the output even when there is no incident light at all.
The saturation region, which corresponds to the tract 8 of the curve in FIG. 2a, is characterized by the fact that whenever an excessive quantity of photons strikes the cell during the acquisition time, as from a certain point onwards even a substantial increment of the number of incident photons will produce only a minimal or zero increase of the output voltage.
Since the response of each individual light-sensitive element is limited, the sensor introduces a first substantial limitation of the response capacity of the still camera to variations of the light intensity.
Just as in the case of traditional still cameras, control of the exposure, obtained by acting both on the size of the diaphragm opening and the shutter timing, makes it possible to regulate the quantity of light that strikes the sensor during acquisition. But when the radiance variation of the scene, i.e., the quantity of light the scene emits per unit area, is too great, even the best regulation of the exposure will not prevent the acquisition of an image with a part of the cells below the threshold or with a part of the cells in saturation or, in the worst case, with the contemporary presence of cells in saturation and cells below the threshold.
On the output side of the sensor, the voltage values generated by each cell are translated into digital values by an analog/digital converter.
The overall memory quantity that the formats most commonly employed for storing digital images utilize for each pixel amounts to 24 bits, of which, eight are reserved for luminosity information (corresponding to 256 possible distinct brightness levels).
The conversion of the voltage values from the analog form to the digital form introduces an error, the so-called quantization noise, derived from the fact that this conversion implies a passage from an infinite number of shadings to a finite number of levels. This process further accentuates the loss of information for the pixels codified with digital values in the neighbourhood of the two extremes of the scale (0 and 255), where small variations of light intensity may become completely cancelled.
The combined effect derived from these facts is that whenever the real scene is characterized by the presence of considerable variations of light intensity, which may be due, for example, to the simultaneous presence of both shadow areas and reflecting surfaces, the image acquired with the digital still camera will suffer a loss of detail. All the regions of the image characterized by high luminosity (reflecting surfaces) will be affected by visible saturation effects, while the regions of very low luminosity (shadow zones) will become uniformly codified with an intensity value “0” or values close to “0”.
Present-day digital still cameras either incorporate automatic gain control (AGC) systems that assure automatic regulation of the sensitivity of the light-sensitive elements or, alternatively, incorporate automatic exposure setting systems that operate in conformity with a multitude of different criteria. Yet another alternative is represented by the possibility of manually regulating the exposure level.
These expedients make it possible to exploit the response of the still camera in an optimal manner or enable the operator to select the part of the real scene that is to be acquired with the greatest amount of detail, but cannot enlarge the dynamic range of the still camera.
The prior art also comprises a technique that employs a combination of a set of images, all representing one and the same real scene but acquired with different exposures, to obtain a final image of a higher quality.
This technique is based on the consideration that the details that would be lost if the scene were to be acquired with a single exposure can be correctly recorded by employing a series of images acquired with different exposures and then selecting from them the particular portions that in each image are reproduced in an optimal manner.
The methods that utilize this technique are based either on the determination of the response function of the still camera or on the determination of the “radiance maps” as the means for obtaining the information needed for selecting regions of the various images and then automatically combining them.
In particular, U.S. Pat. No. 5,828,793 (Mann) describes a method of combining images that consists of determining the response function of the still camera and combining the source images by constructing a weighted average of these images pixel by pixel. The weighting coefficients used for the weighted average are calculated on the basis of the response function.
Although the method proposed by Mann is capable of identifying the best-exposed portions of the source images, in the combination of the various images it produces visible artifacts due to the sudden passage from one image to the other.